Mossberg 500a Dating

[WideAwake]

Got a Nice Yard sale Find. Mossberg 500a with 28"&30" barrels & side scope mount & cheap scope $130 total. I want dress it with a polymor forend But I hear there is 2 different size fore grips for these guns.. anyone Know how to date this gun.
Serial # code???

Thanks in Advance

[richbug]

The difference in the forends.... Early ones had one action bar, later ones had two.